Experience indicates that yttria-alloyed tetragonal zirconia polycrystal (Y-TZP) ceramic offers many advantages over other engineering materials in terms of structural applications. Y-TZP is one of the toughest ceramics known today. Although the toughness is achieved at the expense of hardness and strength, Y-TZP is still far superior to many engineering materials as far as wear, abrasion, and corrosion resistance properties are concerned. Alumina toughened zirconia (ATZ), comprising tetragonal zirconia and alumina is a tough composite ceramic, which potentially can be useful as Y-TZP for many structural applications. These materials, Y-TZP and ATZ, have been successfully used for high precision punches, dies, slitter knives, hydro-dynamic bearings and a multitude of machine components. Unfortunately, high volume manufacturing processes for these materials using conventional dry pressing, gel-casting, or cold isostatic pressing is cost-prohibitive. Also, net-shape production of complex shapes using the above processes is difficult. Injection molding, however, is a process that can be applied to manufacture a large volume of complex shaped parts in a cost-effective way.
Injection molding of fine inorganic powders comprising sub-micron particles, such as Y-TZP, poses many manufacturing problems. It also becomes very difficult to obtain uniform properties of the sintered parts manufactured by an injection molding process if the particle size of powders have a bi-modal distribution. Specifically, ATZ, has a bi-modal particle size distribution. In order to achieve superior properties of Y-TZP, chemically pure zirconia needs to be alloyed with a stabilizing agent(s), and the particle size needs to be maintained at a sub-micron level, preferably at or below 0.3 μm.
Injection molding complex shaped parts using inorganic powders is not a trivial process. As stated earlier, the injection molding processes for ceramics and metals become extremely challenging when the particle size is in the sub-micron range. Ceramic powders are mixed with primary binders, plasticizers, and processing aids so that a highly viscous material flows to the mold and forms self-supporting solid parts that have sufficient green strength, enabling one to handle them prior to sintering. Improper mixing, along with improper compounding formulation, will cause segregation of the inorganic particles or separation from the organic binder components, as the shear forces act upon the mixture during the injection molding process.
Plasticizers are stable unreactive materials that are added to primary binder(s) to make the compounded product more flexible. Phthalates, adapates, laurates, and oleates are some examples of plasticizers that are compatible with water soluble primary binders. Likewise, a processing aid is added to the binder system to improve the performance during the injection molding process, by modifying the cohesive forces between the binder and ceramic particles. A processing aid reduces the viscosity of the mixture, improves the flow characteristics during molding and eases the release of injection molded parts from the mold. Another important function of a processing aid is to uniformly distribute the binder components throughout the mixture and to enhance the dispersion characteristics of the ceramic particles.
In order to meet some of the requirements of injection molding inorganic powders, preferably ceramic powders having sub-micron particle size, references are made in the prior art whereby compounding, along with mixing, includes adding low melting point wax and some processing aids. Methylcellulose polymers and other high molecular weight polymers are also used as binders in injection molding metallic and ceramic parts. However, such formulations exhibit a multitude of problems in molding complex shape parts. One of the biggest drawbacks with most of the injection molding binder system formulations for metal and ceramic powders is that a lengthy debinding process is necessary to remove the excess binders from the injection molded green parts, prior to sintering, to obtain the finished part. The lengthy debinding step generally includes removing the organic binders, either thermally, or by using chemical solvents. The debinding step may produce a toxic effluent (gas or liquid), thus posing concerns for the environment.
Other binder system formulations described above have not proven to be effective in compounding Y-TZP particles, probably because of the extremely fine particle size associated with Y-TZP. Also, it is difficult to manufacture complex net-shape Y-TZP parts using other formulations because internal cracks and defects often develop during the debinding step. One source of cracking and warping can be attributed to using gel forming material, such as agaroids like agar, agarose and mixtures thereof, in the compounding process.
Consequently, there is a need for an improved method of compounding extremely fine inorganic particles and manufacturing complex net-shaped ceramic parts.